Mold and Method for the Manufacture of Holders Such as Tub-Shaped Containers

ABSTRACT

This invention relates to a mold for the manufacture of holders, comprising at least two mold halves moveable relatively to each other in a first direction of movement and a mold cavity, the mold being substantially defined by a female part and a matching male part, while at least the female part and/or the male part is arranged so as to be moveable such that within the mold, the male part and/or the female part can be brought into a first position wherein between a forward end, facing, in a first direction of movement, towards the mold cavity, and an opposite bottom part of the female part, there is a relatively large, first distance, and a second position wherein between said forward end and said bottom part there is relatively small second distance.

The invention relates to a mold for the manufacture of holders. Theinvention further relates to a method for the manufacture of holders.

As a rule, when holders such as tub-shaped containers are injectionmolded, a mold is used wherein plastic is forced from the bottom surfaceinto a longitudinal wall. With small wall thicknesses and/or long flowpaths and/or a large projected surface and/or low melt plastics,particularly high closing pressures and injection pressures need to beused, particularly if a multi-cavity mold is used. As a result thereof,especially bottom surfaces of such holders are of relatively thickdesign.

The invention contemplates a mold for manufacturing holders with which,with relatively low closing pressure and/or low injection pressure,relatively thin-walled holders can be injection molded. To that end, amold according to the invention is characterized by the features ofclaim 1.

By using a moveable male and/or female part, the plastic can beintroduced into the mold cavity with relatively little pressure. Thefact is that since the space into which the plastic is introduced isinitially kept relatively large during use by creating additional spacebetween the male and female part, initially, the plastic needs to beintroduced over a smaller distance, while, moreover, the counterpressure as a result of flow resistance is relatively low. Only when theplastic has been introduced at least virtually completely, the two partsare moved relatively to each other, towards each other, so that theplastic is moved further into the mold cavity. It is preferred that themold is closed before the plastic is introduced, so that the plastic isprevented from flowing away inadvertently.

As the above-mentioned relatively large distance is located between theforward end of the male part and the opposite bottom surface of thefemale part, the advantage is achieved that, in a simple manner,additional space can be created, since the distance is locatedapproximately parallel to the closing direction of the mold, at leastthe ejecting direction of the ready products. Preferably, the plastic isthen introduced in the space between this forward end of the male partand the opposite bottom surface of the female part, so that minimumresistance is experienced and a favorable flow pattern is provided.

It is further preferred that the distance between the male and thefemale part in the first position is selected such that virtually theentire volume of plastic required for the holder can be introduced intothis space between the forward end of the male part and the oppositebottom surface. As a result, a particularly advantageous initial flowpattern is obtained.

With a mold according to the invention, in a particularly advantageousmanner, holders can be manufactured with a truncate shape. Herein,truncate is understood to include a holder with (a) slightly inclininglongitudinal wall or walls and a bottom, such as frustoconical ortruncate beam-shaped, which latter has a substantially rectangular baseshape with a longitudinal wall inclining substantially outwards from abottom surface. The first-mentioned shape can, for instance, be found incups and the like, the latter in, for instance, butter tubs, ice-creamcontainers and the like. However, these examples should not be construedto be limitative in any way.

With a mold according to the invention, in particular holders can beformed with a relatively thin bottom and longitudinal wall, for instancethinner than matching the melt-flow index for a similar product withconventional molding. In particular, a bottom can be obtained which isrelatively thin, so that material can be economized on. Furthermore, amold according to the invention can simply be arranged for in-moldlabeling, as the pressure of the plastic upon injection is relativelylow, especially when the plastic is introduced into a bottom surfacewhich is not to be labeled.

When using a mold according to the invention for in-mold labeling, whileboth the bottom and the longitudinal wall or walls are to be providedwith a label, it is preferred that the plastic be introduced through thepart against which the label is not provided. With a customary holder,this will mean that the plastic is introduced through the male part intoa bottom forming part of the mold cavity, while the label is inserted atthe opposite side thereof in or over the female part. As a result, theoutside of the holder is provided with a label, while damage to anddisplacement of the label is prevented.

With a mold according to the invention, the design of the male andfemale part is preferably such that in the first position and the secondposition, the distance between the female part and the male partsubstantially only varies in or near the bottom forming part of the moldcavity, between the forward end of the male part and the bottom surfaceof the female part. As a result, a simple mold is obtained withfavorable load of the different parts.

Preferably, the male part and/or the female part is biased in the secondposition. As a result, it will have to be actively brought into thefirst position, and it will automatically be moved to the secondposition. A pressure member can, for instance, be used in the oppositemold part for pushing the respective male or female part into the firstposition. Such a pressure member can, for instance, be a stripping plateor stripping ring or the like, which can also be used for forcing aready product off the male part or from the female part. The respectivepart, biased in the second position, is preferably biased in the secondposition by simple spring means, for instance a gas filled spring means.In a further advantageous embodiment, during use, the respective part,biased in the second position, is moved by the injection pressure of theplastic to the first position and only when the injection pressure fallsis it pushed back to the second position by the spring means or thelike.

In an alternative embodiment, naturally, also the respective biased maleand/or female part can, conversely, be biased in the first position and,with the aid of drive means such as a screw spindle, piston-cylinderassembly or the like, be moved to the second position so that, forinstance, more pressure can be obtained or a more controlled, forinstance, faster or more uniform pressure build-up can be obtained.

With multi-cavity molds according to the invention, the advantages oflower injection pressure and, in particular, lower closing pressure areachieved to an even larger extent.

The invention further relates to a method for manufacturing holders andsuch hollow products characterized by the features of claim 14.

With such a method, thin-walled products can be manufactured rapidly andwith low pressures. Such a method can be particularly advantageouslyused for in-mold labeling products, for multiple products and forproducts with specific properties such as transparency, or from specialMaterials, for instance low melt plastics such as PET, PC and the like,or crystalline plastics, in particular when these are to be, becomeand/or remain transparent. PET is a clear but non-limitative examplethereof too.

Further advantageous embodiments of a mold and method, and of holders tobe obtained therewith are given in the subclaims. In clarification ofthe invention, as example, exemplary embodiments of a mold, method andholder according to the invention will be further elucidated withreference to the drawing. In the drawing:

FIGS. 1 and 2 show a frustoconical and a truncate beam-shaped holderaccording to the invention;

FIG. 3 shows, in cross-sectional side view, a part of a first embodimentof a mold according to the invention;

FIGS. 4A-C schematically show, in cross sectional side view, a moldaccording to the invention, in three steps of a method according to theinvention; and

FIG. 5 schematically shows, in cross-sectional side view, a moldaccording to the invention, in an alternative embodiment.

In this description, identical or corresponding parts have identical orcorresponding reference numerals. As examples of holders according tothe invention a cup and a butter tub are given, as frustoconical holderand as truncated beam-shaped holder. Naturally, many different shapescan also be manufactured according to the invention with a moldaccording to the invention.

FIG. 1 shows a cup 1, FIG. 2 a butter tub 2. Both are holders 3 with arelatively small wall thickness, for instance some tens of millimeters,and have an internal space 4 which is relatively large. The holders 3are relatively high in relation to the wall thickness. Preferably, thebottoms 5 of these holders 3 have a thickness which is approximatelyequal to or smaller than the wall thickness of the walls 6. However,these thicknesses can be selected to be different and can, for instance,also taper over the wall and/or bottom. The holders can be transparent,opaque or completely non-transparent.

In FIG. 3, schematically, a mold 10 is shown for forming a holder, forinstance according to FIG. 1 or 2. This mold 10 comprises a first part11 and a second part 12, moveable relative to each other. In theexemplary embodiment shown, the second part can be moved actively, aswill be elucidated further with reference to FIG. 4, for opening andclosing the mold. A supply device 13 is provided in the first part 11.

The first part 11 comprises a male part 14, which is substantiallyformed by a frustoconical or truncate block-shaped part 15, fixedlyconnected to a first base plate 16. In the base plate 16, a stripperring 17 or such pressure means is provided which extends around the wideend of the part 15 and is moveable in the direction S with the aid of anejector plate 18. In the position shown in FIG. 3, the ejector plate 18and hence the stripper ring 17 are shown in a retracted, secondposition.

The second part 12 comprises a female part 19, which is substantiallyformed by a block 20 having a cavity 21 therein which corresponds as toshape to that of part 15 and has dimensions such that, as shown in FIG.3, with the mold 10 in the closed position and the stripper ring 17retracted, between the male part 14 and the female part 19, a moldcavity 22 is left open, at least defined which has, substantially, theshape of the desired holder 3. The block 20 is included in, forinstance, a cylindrical space 23 in the second part 12, in which it canbe shifted in the direction S. Between the end of the block 20 facingaway from the first part 11 and the closed end 24 of the space 23, achamber 25 is formed, filled with, for instance, gas or oil or a similarmedium for forming a pressure chamber. Upon movement of the block 20 inthe direction of this end 24, in the chamber 25, an increased pressurewill occur, biasing the block 20 in the direction of the first part 11and hence in the direction of the second position shown in FIG. 3.

In this mold 20, a holder 3 can be formed by introduction of plastic insubstantially liquid, at least, moldable form. To that end, steps can betraversed as shown in FIGS. 4A-C.

In FIG. 4A, the mold 10 is shown in opened condition. The block 20 hasbeen pushed by the medium into the chamber 25 into the second position,maximally forward (towards the right in FIG. 4). The stripper ring 17 inthe first part 11 is moved outwards (towards the left in FIG. 4) withthe aid of the ejector plate 18.

In FIG. 4B, the mold 10 is shown in closed condition. The stripper ring17 is held in the position moved in outwards direction, shown in FIG.4A. The stripper ring 17 then abuts against the block 20, so that theblock 20 is moved backwards, in the direction of the end 24, therebyreducing the volume of the chamber 25 and compressing the mediumtherein. As a result, pressure is exerted on the stripper ring 17. FIG.4B shows the second position. As clearly appears from FIG. 4B, theforemost end 26 of the male end 15 (facing the left in FIG. 4) is at arelatively large distance D from the bottom surface 27 of the cavity 21.This distance D is, in particular, larger than the distance between themale part 15 and the wall of the cavity 21 in the female part 19.Between this end 26 and the bottom surface 27, a bottom forming space 29is formed into which the plastic is introduced via the supply means 13(not shown in FIG. 4). In the second position, shown in FIG. 4B, thevolume of the bottom forming space 29 is relatively large, for instanceapproximately equal to the volume of material required for the entireholder. Consequently, the plastic can be introduced into the mold cavity22 rapidly and in a simple manner, with little counter pressure.

After the plastic has been, at least for the larger part, introducedinto the mold cavity 22, at least in the bottom forming cavity thereof,the ejector plate is moved backwards (to the right in FIG. 4), to afirst position as shown in FIG. 4C. The stripper ring 17 is then nolonger retained (or actively pulled back) so that the block 20 can bepushed by the medium into the chamber 25 to the first position (to theright in FIG. 4C). The bottom forming space 29 is then considerablyreduced. The distance D1 between the end 26 and the bottom surface 27 isreduced to the desired wall thickness, for instance equal to the wallthickness of the wall 6 of the holder. The plastic is then pressed, forthe larger part, from the bottom forming space 29, between the male part15 and the wall of the cavity 21, for completely filling the mold cavity22 and hence forming the desired product 3.

After the holder 3 has been formed and, optionally, has cooled down, themold can be opened again to the position shown in FIG. 4A, while theejector plate 18 and hence the stripper ring 17 are moved forwards,thereby forcing the ready product from the male part 15.

In FIG. 5, schematically, an alternative embodiment of a mold 10according to the invention is shown, wherein two male parts 15, having asubstantially cylindrical or slightly conical shape, are attached to amoveable base plate 40, which base plate 40 is moveable in a chamber 41formed in the first part 11 of the mold 40. In the second part 12, fixedcavities 42 are provided with stripper rings 17 around them, biased inthe direction of the base plate 40 by springs 43. Supply means 13terminate in the cavities 42 through the second part 12. A supply duct44 with pressure pump 45 and a discharge duct 46 with a suction pump 47open into the chamber 41, so that the pressure in the chamber 41 can beactively regulated. With this, it can be regulated whether the stripperrings 17, under the influence of the springs 43, can force the baseplate 40 with male parts 15 backwards in the direction away from thesecond part 12 or not, or the base plate can be actively withdrawn fromthe second part or, conversely, be moved thereagainst. With this, eachtime, the distance D between the end 26 of the male parts 15 and thebottom surface 27 of the cavities 21 can be set. Also, the pressure inthe chamber 41 can be set to be such that, due to the pressure of theplastic flowing into the mold cavities 22, the base plate can be pushedaway and only be moved back if the injection pressure falls after allplastic has been introduced. Optionally, the springs can then be omittedor be replaced by controlled gas springs with which the stripper rings17 can be actively moved for ejecting the holders and/or abuttingagainst the base plate 41.

Before the mold 10 is closed, a label can be inserted, for instance overor in the cavity 21 or on the male part 15, for in-mold labeling. InFIG. 4A this is symbolically shown by the chain-dotted line L.

Naturally, all combinations of parts of molds according to the inventioncan be used.

The invention is not limited in any manner to the embodiments as shownand described. Many variations thereon are possible within the frameworkof the invention. Naturally, the pump 45 and 47 can be combined in adouble acting pump.

For instance, other means can be used for bringing and/or holding amoveable male part and/or a female part in the first and/or secondposition, such as screw spindles, piston-cylinder assemblies, electricmagnets and such means known per se. Other numbers of mold cavities canbe provided, single or stack molds. The movements of moving parts can beactively controlled. All sorts of other shapes of holders can be formedwith a mold or method according to the invention.

These and many comparable variations are understood to fall within theframework of the invention.

1. A mold for manufacturing holders, comprising at least two mold halvesmoveable relative to each other in a first direction of movement and amold cavity, the mold cavity being substantially defined by a femalepart and a matching male part, while at least the female part and/or themale part is arranged so as to be moveable such, that within the mold,the male part and/or the female part can be brought into a firstposition wherein between a forward end, facing, in a first direction ofmovement, towards the mold cavity, and an opposite bottom part of thefemale part, there is a relatively large, first distance, and a secondposition wherein between said forward end and said bottom part there isa relatively small second distance.
 2. A mold according to claim 1,wherein at least one inlet opening is provided in or near a spacebetween said forward end of the male part and said bottom part of thefemale part.
 3. A mold according to claim 2, wherein said inlet openingis provided in the male part.
 4. A mold according to claim 1, whereinthe male part is frustoconical and the female part has a correspondingshape, such that with the male and female part in the second position,between the two parts, there is a virtually constant distance.
 5. A moldaccording to claim 1, wherein the male part is truncate block-shaped andthe female part has a corresponding shape, such that with the male andfemale part in the second position, between the two parts, there is avirtually constant distance.
 6. A mold according to claim 1, wherein atleast the male part or the female part is biased in the second position.7. A mold according to claim 1, wherein a pressure member is providedfor actively keeping the male part and/or the female part in the firstposition, which pressure member is retractable for having the male andfemale part move from the first position to the second position.
 8. Amold according to claim 7, wherein the pressure member extends around atleast a part of the male part and, with the mold closed, can force, atleast hold, the female part in the first position.
 9. A mold accordingto claim 8, wherein the pressure member is designed as stripper ring.10. A mold according to claim 1, wherein at least one moving male and/orfemale part is forced into the first position by spring means, inparticular gas-filled spring means.
 11. A mold according to claim 10,wherein the spring means are designed such that the respective male orfemale part can be pushed away to the first position by plastic flowinginto the mold cavity, and, when the injection pressure of the plasticfalls, is forced to the second position by the spring means.
 12. A moldaccording to claim 1, wherein the or each moveable part is arranged formovement to a product forming position after the mold has been closed.13. A mold according to claim 1, wherein the mold is of multi cavitydesign, while for each mold cavity at least one moveable part isprovided.
 14. A method for manufacturing plastic, thin-walled products,wherein in a closed mold, according to claim 1, in a mold cavity, a maleand/or female part is brought to a position moved backwards, while orwhereupon plastic is introduced into the mold cavity, whereupon the malepart and/or the female part is brought to a position moved forward,while forcing at least a part of the plastic away such that the entiremold cavity is filled, while in the position when moved backwards,between a forward end of the male part in the direction of movement anda bottom of the female part, there is a larger distance than in theposition when moved forward, while the distance between the remainingparts of the male and female part remains virtually the same.
 15. Amethod according to claim 14, wherein as a male part a substantiallyfrustoconical or truncate block-shaped part is used, while a female partwith a cavity having a corresponding form is used.
 16. A methodaccording to claim 14, wherein a holder is formed with a bottom surfacehaving a thickness which is approximately equal to or is preferablysmaller than the thickness of a longitudinal wall extending awaytherefrom.